1. Field of the Invention
This invention generally relates to propulsion systems which operate in a reduced-gravity environment, and more particularly to a system and method for controlling the propulsion system of a spacecraft to achieve a desired position and/or orientation.
2. Description of the Related Art
As the space programs of developed countries mature, satellites and other forms of spacecraft become more widely deployed. The extent of their use is largely driven by commercial demand. For example, the vast communications needs of the modern world have made geosynchronous satellites the most common manmade object in space today. One satellite of this type is the A2100 spacecraft manufactured by Lockheed Martin. The A2100 offers Ka-band/broadband services, fixed satellite services in a hybrid Ku-C-band payload configuration, and high-power direct broadcast services using the Ku-band frequency spectrum among others. Because of its versatility, the A2100 is often the satellite of choice by telecommunications providers.
In order to attain a desired orbital position or orientation (e.g., attitude), many forms of modern spacecraft are equipped with propulsion systems. These systems typically use liquid propellants to fuel an arrangement of thrusters on the spacecraft body. Liquid propellants are preferable because their flow into the thrusters can be regulated using valves and thus activation of the thrusters can be controlled with relative ease and precision.
As shown in FIG. 1, liquid propellant 1 is typically housed within a spherical tank 2 which is made of titanium for high strength and low mass and which is large enough to hold one hundred kilograms or more of, for example, hydrazine. From the tank, a system of plumbing 3 carries the propellant (which in this example is called mono-propellant since there is only one liquid) through filters and valves. When the valves are opened, hydrazine squirts onto a hot catalyst, which provokes the hydrazine to decompose rapidly, releasing heat and expanding in the process. The hot, expanding gases force themselves out through a controlled number of thrusters 4. The act of expelling mass in this manner creates thrust which causes the spacecraft to achieve a desired orbital position and/or orientation.
In order to ensure propellant properly flows into the thrusters in a reduced-gravity environment, the propellant tank must be pressurized. In a mono-propellant system such as described above, a flexible diaphragm 5 is used within the tank to separate the propellant from an area 6 (commonly referred to as ullage) that is pressurized with helium. The introduction of helium into the tank keeps the propellant pressed against an outlet, thereby providing the force required to drive the propellant through the plumbing when the valves open. In most mono-propellant systems, a small tank of helium 7 is used to re-pressurize the ullage as necessary to ensure the proper flow of propellant over time.
Some spacecraft are fueled with two liquid propellants held within separate tanks. These so-called bi-propellant systems manage their fuel loads differently. For example, both propellants are pressurized with helium supplied at a regulated pressure from a dedicated high-pressure helium tank. Unlike mono-propellant systems, no diaphragm is used to separate the propellants from the helium. Instead, each tank is equipped with a surface tension management device which causes propellant to flow during thruster firing. U.S. Pat. No. 5,251,852 discloses a bi-propellant system of this type.
Conventional propulsion systems have proven to be costly to operate and prone to failure. The use of helium to re-pressurize the propellant tanks, for example, has proven to be risky. Conventional systems have also demonstrated slow response times which have made them unreliable, especially over long-term use.
In view of the foregoing considerations, it is clear that there is a need for an improved system and method for controlling a propulsion system, and more specifically one which does so more accurately, more cost effectively, and with lower risk compared with conventional systems.
The present invention is an improved method for controlling a propulsion system of a space-borne object (e.g., a satellite) more accurately, more cost effectively and with lower risk compared with conventional systems. The method is based on the principle that temperature in the propellant tank is directly proportional to the pressure therein. This pressure, in turn, greatly influences the performance efficiency of the propulsion system.
One problem the invention addresses is low pressure in the propellant tank. When deployed in space, pressure in this tank tends to wane over time and thus the performance of the propulsion system commensurately becomes affected. The system and method of the present invention offsets these effects by preventing the tank pressure from falling below a certain level, thereby ensuring that the propulsion system will be maintained at desired performance and efficiency level. In accordance with one embodiment, the pressure of the propellant tank is measured and then compared to a predetermined criteria. When determined to be insufficiently low, the pressure in the propellant tank is increased by effecting a temperature increase in the tank. The temperature increase may be accomplished through a heater which is mounted on or proximate to the propellant tank and which is operated either automatically under control of an on-board processor or manually through commands transmitted from a ground station. Preferably, the temperature is increased by an amount that will re-pressurize the tank back to a level that will ensure proper functioning of the propulsion system.
In accordance with other embodiments of the invention, thruster performance data other than tank pressure is monitored from the satellite during a test maneuver. This thruster performance data includes any one or more of thruster on-times, power level of a power conditioning unit (PCU), and temperature of the propellant tank. This data is then evaluated to provide an indication of the tank pressure and thus whether re-pressurization is warranted through a temperature increase. If desired, the re-pressurization may be effected through the combined approach of increasing the temperature of the propellant tank and introducing helium gas therein.
In accordance with still another embodiment of the invention, the temperature of the propellant tank is raised, first, by heating the helium gas and, then, by introducing the heated gas into the propellant tank by an amount that will achieve a desired pressure increase. By re-pressurizing the propellant tank in any of the above ways, spacecraft propulsion systems will be assured of consisting operating with a desired performance efficiency.